1. Field of the Invention
Example embodiments of the present invention relate to an organic thin film transistor (hereinafter also referred to as an “OTFT”) including a fluorine-based polymer thin film. Various example embodiments of the present invention relate to an organic thin film transistor including a gate electrode, a gate insulating layer, an organic semiconductor layer, a source electrode, and/or a drain electrode formed on a substrate, wherein a fluorine-based polymer thin film may be formed at the interface between the gate insulating layer and the organic semiconductor layer.
2. Description of the Related Art
Various thin film transistors (TFTs) currently used in displays may include an amorphous silicon semiconductor, a silicon oxide insulating film and/or metal electrodes. With recent developments in various electrically conductive organic materials, research has focused on developing organic TFTs using organic semiconductors. Since the organic thin film transistors (OTFTs) developed in the 1980's may have advantages in terms of superior flexibility and/or ease of processing and fabrication, they are now under investigation for application to display devices, for example, E-ink devices, organic electroluminescence devices and liquid crystal displays (LCDs). Organic semiconductors may also be used in a wide variety of applications, e.g., functional electronic and optical devices, on account of their various synthetic processes, ease of molding into fibers and films, superior flexibility, and/or low fabrication costs. When compared to silicon transistors using amorphous Si, OTFTs using an organic semiconductor layer made of conductive organic molecules may have some advantages. For example, a semiconductor layer may be formed by printing processes at ambient pressure instead of conventional chemical vapor deposition (CVD) processes, such as plasma-enhanced chemical vapor deposition (CVD), and optionally, the overall fabrication procedure may be accomplished by roll-to-roll processes using plastic substrates.
Despite these advantages, OTFTs may encounter problems including low charge carrier mobility, high driving voltage and/or high threshold voltage, when compared to amorphous silicon TFTs. Charge carrier mobility of 0.6 cm2·V−1·sec−1 has recently been achieved in pentacene-based OTFTs, potentially increasing the use of OTFTs in certain applications. However, the mobility still may be unacceptable for practical TFT applications. In addition, drawbacks of pentacene-based TFTs may be a high driving voltage (≧100V) and/or a high sub-threshold voltage 50 times than that of amorphous silicon TFTs.
On the other hand, other conventional art discloses organic thin film transistors with reduced driving voltage and/or threshold voltage using high dielectric constant (κ) insulating films. According to this conventional art, the gate insulting films may be composed of inorganic metal oxides, for example, BaxSr1-xTiO3 (barium strontium titanate (BST)), Ta2O5, Y2O3, TiO2, etc., and ferroelectric insulators, for example, PbZrxTi1-xO3 (PZT), B4Ti3O12, BaMgF4, SrBi2(Ta1-xNbx)2O9, Ba(Zr1-xTix)O3 (BZT), BaTiO3, SrTiO3, Bi4Ti3O12, etc. In addition, the gate insulting films may be formed by chemical vapor deposition, physical vapor deposition, sputtering, and/or sol-gel coating, and may have a dielectric constant above 15. The lowest driving voltage of the OTFTs may be reduced to −5V, but the highest charge carrier mobility still may be unsatisfactorily at 0.06 cm2·V−1·sec·−1. Furthermore, because most of the fabrication may require a high temperature of 200-400° C., the range of applicable substrates may be limited and common wet processes, for example, simple coating and printing, may not be easily applied to fabricate such devices.
Other conventional art suggests the use of polyimide, benzocyclobutene, photoacryls and the like as materials for organic insulating films. However, because these organic insulating films may exhibit unsatisfactory device characteristics over inorganic insulating films, they may be unsuitable to replace inorganic insulating films.
Attempts have been made to use double-layer or multilayer gate insulating layers in order to improve the performance of thin film electronic devices, for example, a multilayer gate insulating layer including two insulating layers made of silicon nitride and silicon oxide, and a double-layer insulating film including two insulating films made of the same material, which may improve the electrical insulating properties and/or the crystalline quality of semiconductor layers. However, because both gate insulating films were developed only for amorphous silicon- and single crystal silicon-based inorganic TFTs and use inorganic materials, they are not suitable for use in the fabrication of organic semiconductors.
As the application of OTFTs has recently been extended not only to LCD displays, but also to driving devices for flexible displays using an organic EL, the OTFTs may be required to have a high charge carrier mobility above 5 cm2·V−1·sec−1, a low driving voltage, and/or a low threshold voltage. Further, insulating films used in OTFTs may require superior insulating properties. Particularly, for simplified fabrication procedures and/or reduced fabrication costs, it may be desirable to fabricate OTFTs on plastic substrates by all-printing and/or all-spin on processes. Under these circumstances, although many studies have been devoted to forming organic gate insulating layers in a simple manner and increasing the charge carrier mobility at the interface between organic semiconductor layers and organic gate insulating layers, satisfactory results have not be achieved.